Thiobacillus sp. 65-29 Raw sequence reads. Thiobacillus sp. 65-29

Denitrifying bacteria can either directly accept electrons from the electrode or indirectly via electron shuttling intermediaries generated by the cathode. In this study, direct electron transfer denitrification (DETD) was stimulated in an electrochemical reactor. Comparative analyses were conducted with a system that generated H2 through water electrolysis. Nitrogen removal efficiencies were much higher in the DETD system, >93% compared to only 20%. Notably, a biofilm formed exclusively on the DETD cathode, predominantly composed of Thiobacillus species, with the majority similar to Thiobacillus strain 65-29. Metatranscriptomics showed that genes from strain 65-29 encoding outer surface c-type cytochrome and porin proteins that could facilitate direct electron uptake, were markedly up-regulated under DETD conditions. Reduction of sulfate (~7 mM) present in the inoculum by the cathode or sulfate reducing bacteria might have also provided a source of electrons for nitrate reduction. In fact, the DETD system exhibited elevated expression of genes involved in thiosulfate (soxABXYZ), sulfite (soeABC), and sulfide (fccAB) oxidation.